Currently practiced approaches for producing plants having agronomically important traits generally rely on conventional plant breeding programs in which plants of a different genotype are generally crossed in order to produce a hybrid with a recognizable agronomically important trait. Such an agronomically important trait may be an easily recognizable morphological characteristic such as fruit size or color, or alternatively traits, which are difficult to evaluate that, may be selected by using indirect selection criteria. One indirect selection criterion for example might be an easily recognized morphological characteristic of the plant which is either genetically linked to the desired trait or a component of the desired trait or contributes to the desired trait.
Many different approaches can be taken to disclose, reveal or have such expressed morphological characteristics in tomato. One of these approaches is the generation of various tomato plant lines in which introgressions of wild tomato were introduced into a cultivated tomato genetic background were generated in efforts to isolate quantitative trait loci or genes underlying the morphological characteristics.
Tomato reproductive development initiates with the termination of the primary shoot meristem into an inflorescence.
Extensive variation in inflorescence complexity is found in the nightshade (Solanaceae) family. In most cultivated tomatoes a sympodial growth system is presented. The sympodial system is composed of superposed branches such that an apparent main axis or stem, comprising successive secondary axes that terminate in a flower, is formed. Such a zigzag branching pattern repeats a few times before terminating, resulting in a cluster of a few flowers. The inflorescences of tomato have been characterized as a cyme or raceme. Cymose inflorescences are mainly determinate. In such a cymose or definite type inflorescence, the growth of the main stem is definite. The main stem produces lateral branches which grow more vigorously than the main axis. As a result of this branching, the plant spreads out above. A cymose branching has several kinds. A uniparous cyme is a cymose type of branching with only one lateral branch produced at a time. It is also known as monochasial or sympodial. It shows two distinct types namely helicoid and scorpioid. A biparous cyme, also known as dichasial, is when two lateral branches develop at a time. In such configuration, the key axis leads to a flower after presenting daughter lateral axis. These kinds of lateral axis as well as other branches also function within the similar way. A multiparous cyme or polychasial is when more than two branches develop at a time. In such inflorescence type, the primary axis leads to a flower after producing numerous lateral branches, which usually forms numerous horizontal flowers and provide an umbel configuration. Racemose inflorescences are indeterminate. Such a racemose or monopodial or indefinite type inflorescence is characterized by a stem that indefinitely grows by the terminal bud. The lateral branches of the main stem are arranged in an acropetal succession (produced successively towards the apex with older branches towards the base and younger ones towards the apex). As a result of this branching the plant appears conical or pyramidal in shape.
According to UPOV guidelines for Tomato Lycopersicon lycopersicum, the inflorescence type may be characterized as mainly uniparous or intermediate (partly uniparous and partly multiparous) or mainly multiparous. The inflorescence of determinate growth type varieties may be characterized by the number of inflorescences on main stem, which is estimated as few, medium, or many (UPOV TG/44/10 Guidelines for the conduct of tests for Distinctness, Uniformity and Stability).
A recent study shows a bifurcation of the meristem into a determinate floral and indeterminate inflorescence meristem (Welty et al., 2007).
Wild tomato species with an inflorescence structure that kept iterating to form compound clusters of flowers are long known. This variant form was given the name “compound inflorescence” to distinguish it from the “normal or regular inflorescence” form. Several mutations are known to affect the structure of tomato flower clusters. These mutations include the jointless (j), blind (bl), anantha (an), flsiflora (fa), single flower truss (sft), self pruning (sp), compound inflorescence (s) and iniflura (uf) loci. These mutations were found to affect inflorescence and floral meristem development by blocking the transition, reverting to vegetative growth or controlling the number of flowers per inflorescence (Allen and Sussex 1996; Dielen act al., 1998; Molinero-Rosales et al., 1999, 2004; Schmitz et al 2002; Lifschitz et al., 2006; Quinet et al., 2006; and, Szymkowiak and Irish 2006).
The mutations that control inflorescence development were often found to be pleiotropic, in that they control other aspects of development such as flowering time and formation of an abscission zone on pedicels. For example in all environmental conditions, the sit mutant flowered significantly later than its corresponding Platense (PI) cultivar. Flowering time of j was consistently delayed compared with Hienz (Hz), and flowering of s mutant was reported to be delayed in winter compared with the Ailsa Craig (AC) cultivar (Quinet et al., 2006). Furthermore, the s mutant was shown to develop highly branched inflorescences bearing up to 300 flowers due to the conversion of floral meristems into inflorescence meristems, resulting in reduced individual fruit weight (Vriesenga and Honma, 1973) Quinet et al., 2006). The studies of Crane and MacArthur noted that simple inflorescence was dominant to compound inflorescence (Crane, 1915; MacArthur, 1928).
The study of Lippman and colleagues, 2008 described the characterization of two genes that affect the structures of tomato flower cluster. The S gene was found to encode transcription factor related to a gene called WUSCHEL HOMEOBOX 9 that is involved in patterning the embryo within the plant seed (in Arabidopsis thaliana). The AN gene was found to encode an F-box protein ortholog of a gene called UNUSUAL FLORAL ORGANS that controls the identity of floral organs. Mutations in both genes were found to transform the well known tomato “vine” into a highly branched structure with hundreds of flowers (Lippman et al., 2008). Three independently arisen alleles of s (s-classic, s-multiflora and Rose Quartz Multiflora) were shown to be responsible for the diversity in tomato inflorescence architecture. However, their effect on yield properties of cultivated varieties was not disclosed.
Most of the efforts in researching inflorescence development mutations were focused on Solanum pimpinellifolium L., LA1589 wild tomato related species. This red fruited species produces inflorescences with an average of 20 flowers per cluster. As stated by the publication of Lippman and colleagues, 2008, the s mutant was originally mapped on the long arm of chromosome 2. It was positioned in the region overlapping introgression lines IL2-3 and IL2-4 of the tomato introgression line map (Eshed and Zamir, 1995). In Eshed and Zamir study, 50 introgression lines (ILs) originating from a cross between the green-fruited species Lycopersicon pennellii and the cultivated tomato cv M82 were identified. Each of the introgression lines was characterized for quantitative traits associated with yield as compared to the parental species (Eshed and Zamir, 1995). It was found that in homozygous state, IL2-3 and IL2-4 lines showed significant reduction in yield and fruit mass values relative to the control (M82). The heterozygous hybrids of IL2-3 and IL2-4, exhibited lower yield values (IL2-3) or non significant increase in the yield value (IL2-4) and lower fruit mass values relative to the control (Eshed and Zamir, 1995).
Thus, in view of this, it would be useful to have a means and method for distinguishing high yield hybrids from low or non significant yield hybrids.
It is furthermore a long felt need to have means and methods of obtaining cultivated tomato lines with elevated fruit yield.